One measuring principle, among a number of contactless measuring methods for ascertaining fill level in a containment, is the travel time measuring method. In the case of the travel time measuring method, for example, microwaves, or radar waves, as the case may be, are emitted via an antenna apparatus, and the radar echo waves reflected on the surface of the medium are received back following a distance dependent travel time of the measuring signals. From half the travel time, the fill level of the medium in a container can be calculated. The echo curve represents, in such case, the received signal amplitudes as a function of time, wherein each measured value of the echo curve corresponds to the amplitude of an echo signal reflected on a surface at a certain distance. The travel time measuring method is divided essentially into two ascertainment methods. The time difference measurement is a first ascertainment method, which ascertains the time, which a broadband wave signal pulse requires for a traveled path, and another widely used ascertainment method is the determining of frequency difference between an emitted, frequency modulated, high frequency signal and the reflected, received, frequency modulated, high frequency signal (FMCW—Frequency Modulated Continuous Wave). In the following, no limitation is made to a particular ascertainment method. Fill level measuring devices working according to this measuring principle are available from the Endress+Hauser under the mark MICROPILOT.
Used for some time in process measurements technology for radiating and receiving microwaves are, among others, group radiator antennas, planar antennas, antenna arrays, horn antennas or parabolic antennas. All these antennas for contactless measurement of fill level have, due to their construction, a certain spatial size and also a, most often, divergent radiation characteristic of the microwave signals, so that measurement in small openings in containments is difficult or, most often, even impossible.
If the antennas are used in a frequency range of above 20 GHz, then dielectric lenses could be used for focusing the electromagnetic waves, or for changing the radiation characteristics of the antenna. Dielectric lenses in small arrangements essentially follow the laws of optics. Thus, with a dielectric lens, the radiation profile of the antenna can be matched to the measurement conditions in the process. Such dielectric lens structures are shown, for example, in German Patent DE 44 12 770 A1 and European Patent EP 0 773 598 A2, and are applied in distance warning devices for motor vehicles.